Sale banners and such are hard coded into themes. They usually appear if, when editing a product, the "selling price" is the sale price and the "compare at price" is the original price. They don't show up unless both those fields are filled out in a certain way:
As mentioned, sale banners are hard coded into themes, so only some themes support them. If you did change the prices and nothing happened, your theme probably does not have sale banners as an option in your the theme.
Karmela @ Shopify
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